The invention relates to a method of locating a terminal in a cellular radio system having in each cell at least one base station communicating with the terminals within its area, and which base station measures channel parameters from a signal transmitted by the terminals, and in which system the coverage area of each base station is modelled by a digital map.
A typical mobile telephone system covers a wide geographical area and comprises several coverage areas, or cells, each being normally served by one base station. A cell size varies greatly, generally according to the volume of traffic within the area. The cells are generally smaller in high traffic density areas than in regions where fewer calls are set up. Irrespective of the cell size, situations often occur in which it is useful to know the geographical location or the direction of travel of a mobile telephone.
Several different methods have been provided in order to determine the geographical location of terminals. The distance between the terminal and the base station communicating with it can be easily determined on the basis of the propagation delay of a signal. One of the most commonly used methods is called triangulation in which three or more base stations measure the signal of the terminal, and the location of the terminal is calculated on the basis of the propagation delay of the signal. However, a problem in the prior art methods, such as the triangulation, is that they cannot detect the location of the terminal very accurately, and, consequently, the location information so obtained cannot be utilized in applications requiring accurate geographical location parameters.
An object of the invention is therefore to provide a solution by means of which a terminal can be accurately located and which solves the prior art problems. This is achieved by the method of the type described in the introduction, which is characterized in that to each point on the digital map is added a model of the channel parameters of a signal to be transmitted from the point, and that the channel parameters which are estimated from the signal of the terminal are compared with the parameters of the different points on the map, and the terminal is located on the basis of the comparison.
The invention also relates to a cellular radio system comprising in each cell at least one base station communicating with terminals within its area, and which base station is arranged to measure channel parameters from a signal transmitted by the terminals, and in which system the coverage area of each base station is modelled by a digital map. The cellular radio system of the invention is characterized in that the system comprises means for maintaining the digital map, in which means information is added about a channel parameter estimate of a signal to be transmitted from each point on the digital map, and that the system comprises the means for comparing the channel parameters estimated from the signal of the terminal with the parameters estimated at different points on the map, and means for locating the terminal on the basis of the comparison.
The invention is based on the idea that when a terminal is situated at different locations within a coverage area, the channel parameters, such as the impulse response, of the signal of the terminal vary depending on the location. Modern computers can profile the geographical location by digital maps to which it is possible to add information about buildings, routes and geography within the area, for example. It is possible to make these maps to the desired accuracy. Naturally, the more accurate the map, the large amount of computer capacity required in its processing. In the solution of the invention, information is added to each point on the map about the types of channel parameters typically included in the signal that is transmitted by the terminal situated at the point. In a preferred embodiment of the invention, the means for maintaining the map and performing calculation are situated in the base station, although they can also be situated in some other parts of the system.
The method and system of the invention provide many advantages. The location of the terminal can be accurately determined. When less accurate prior art localization methods are used as initial values, the accurate location of the terminal can be determined rapidly by the method of the invention. The accurate location information of the terminal so obtained can be utilized in applications requiring accurate geographical location parameters.